Let's look at the project and how Angular gets in the picture. All the relevant files at this stage on exist inside the src folder.
Read more about the files generated by Angular-CLI in Appendix 1: Generating a new project.
Open the file index.html. The content that is rendered in the browser's window is everything you see inside the <body> element. All you can see there now is another, non-HTML element: <todo-root>. This element is a actually an Angular Component, defined in the file app/app.component.ts with the class named AppComponent. (We'll take a look at it in the next chapter).
Note: If you created the project without the flag --prefix todo or -p todo then the element you'll see will be <app-root>. app is the default prefix for the project's component selectors. You can change the configuration in the file angular-cli.json.
Angular can be defined in many ways. One of them is JavaScript code which runs when the application is presented in the browser. All the code you will write - components, modules, services, etc. will be recognized by Angular. Angular will perform actions accordingly. For example, components you will write and use will be compiled to JavaScript functions. These functions insert the component content into the DOM - the Document Object Model which the browser uses to show the application. That's how you'll see the component you created on the screen.
So <todo-root> is not an HTML element, it is an Angular Component. When the application is ready, the content of the component is inserted in place of the <todo-root> tag.
Inside the <todo-root> tag you see the text "Loading...". Everything inside the tag will be rendered by the browser after Angular compiles the application. When Angular is done, you will no longer see "Loading..." on the screen. Instead, you will see the <todo-root> component and its content, which is "todo works!".
Angular needs us to define what we want it to compile. For this we define Angular Modules, or ngModules, which are like packages of related things. These packages can include components, services, directives, pipes and other ngModules. We already have a root ngModule defined for us in the file app/app.module.ts. Let's take a look at this file.
The last line in the file defines a JavaScript class:
export class AppModule { }export is a reserved word in JavaScript which tells that whatever is defined after it should be exposed to other files which import this one using the import statement. You can see examples to classes imported from other files at the first lines in this file.
The class AppModule is empty. It will get its functionality from Angular, which will identify its role by the code preceding this line, starting with @ngModule({.
Every entity in Angular (ngModule, components, services, directives and pipes) is just a class with a decorator. The decorator tells Angular what is the role of this class.
@ngModule({...}) is a decorator. A decorator is just a function. When using it we put @ before its name. This way it becomes a decorator: it looks at what is written right after the function call and receives it as an argument. Decorators usually do something with what they decorate. In this case, for example, ngModule receives the AppModule class and adds to it methods and members that later on will be used by Angular. This way, Angular will recognize that this class represents an ngModule.
What we pass into the decorator function is used by Angular to decorate the class. You can see we pass an object with members, each member is a list of other classes. We'll explain shortly what each member represents.
declarations: a list of Angular things that are relevant in this module. They may use each other (for example, a directive used in a component). We pass here only one component - AppComponent, because that's all we have in our application right now.
imports: a list of other ngModules which are needed for this module. For example, we may use things from FormsModule - directives and services, inside the AppComponent.
providers: a list of services which will be provided at the application root. A service is also a class, and by providing it a single instance (singleton) is created for the whole application. We will talk about services later on.
bootstrap: this member is relevant only to the root ngModule. You will not find it in the modules in the imports list for example. It tells Angular which component should be used as the root component of the application. Every component can use other components in its template. We have one root component that starts the whole structure. So we actually get a tree structure of the components that build our application. In this case, the root component is AppComponent (with the selector todo-root). We saw it used in index.html as the only component inside the <body>.
How does Angular know that the AppModule is the root ngModule? This is defined in the file main.ts in the last line:
platformBrowserDynamic().bootstrapModule(AppModule);We bootstrap our root ngModule to a renderer. This way we tell Angular what ngModule to use as the starting point of our application. And we also choose a renderer: platformBrowserDynamic. It knows how to take our code and add the relevant data (elements, attributes, etc.) to the browser's DOM.
We can use a different renderer at this point, for example one that renders to Android or iOS native elements! We just need a renderer that knows to take our templates (which use HTML notations) and JavaScript code, and create native mobile application elements. An example to such a renderer is NativeScript by Telerik.
There are even renderers to Arduino, with which you can connect sensors, buttons, LEDs and other hardware to your application! You can see a great example for this here: Building Simon with Angular2-IoT
We've seen how we tell Angular where and how to start its work, how we define the root module and the root component, and how we use the root component.
In the next chapter we'll see how a component is defined in Angular.